Since many optical systems, optical components and samples to be analyzed, exhibit polarization dependent behavior, it is desired to have optical beams that are depolarized, before applying the beam to such samples, optical elements, or detectors. Therefore, there is a need to create an optical element that can substantially depolarize a beam of light, regardless of the size of the beam, its center wavelength, optical linewidth, angle of incidence (or numerical aperture), or the coherence of the beam.
Current solutions for depolarization of light beams are limited by either being heavily depended on the beam being spectrally broadband and large in size (such as the Lyot depolarizer), or being an effective depolarizer only in specific wavelength or small range of wavelengths (such as US 2008/0049321A1). In one example, of an optical system doing spectral analysis of a sample (such as a spectrophotometer), since a sample can be polarization sensitive, it is desired to have the beam applied to the sample depolarized, while at the same time to have the beam be as narrowband as possible, in order to obtain spectral resolution. Additionally, a system such as a spectrophotometer performs the measurement in multiple wavelengths, by scanning the center wavelength of the probe beam over a range of wavelengths. If one was to use a Lyot depolarizer in such a system, the performance would be limited to only beams with at least a ten nanometers of optical bandwidth, as the currently existing depolarizers can only depolarizer large bandwidth, achromatic beams, as described in U.S. Pat. No. 3,433,553A. Additionally, the beam would have to be quite large in size (>6 mm), in order for the currently existing solutions to work, which imposes many limitations on the optical design, and efficiency of detection by the optical detector.
If instead one would choose the depolarizer described in US 2008/0049321A1, the depolarizer, by being a half-wave waveplate, would be limited to work only in one specific wavelength, or in a narrow range of wavelengths, due to the nature of having one monolithic birefringent layer, which would make it an effective depolarizer in only a small range of wavelengths. Additionally, by being a half waveplate, this type of depolarizer outputs a plurality of linearly polarized beams, with different fast axis orientation. But since all the parts of the beam are linearly polarized, its effectiveness as a depolarizer is not perfect. This type of depolarizer is not suitable for such applications that require a large spectral range of operation. In the example above, of a spectral scanning device such a spectrophotometer, the said invention would make the device effective only on a small range of wavelength, of a couple of hundreds of nanometers at most, while the spectrophotometers would usually be used to scan over wavelength ranges of a couple of thousands of nanometers.
Previous work includes the Lyot and wedge type depolarizers, which is the most common commercial products but has significant limitations to the beams it can depolarize. Other proposed solutions (but not yet seen commercially) including scattering depolarizers which cause negative side effects to the beam, temporal depolarizes that change the polarization properties over time, and are limited only to applications with large time scales. Fiber optic depolarizers, which are limited for use only in situations where the beams are confined to a fiber optic delivery system. Lastly, a type of depolarizer that is made of an array of segments with different properties, have been suggested by US2008/0049321A1, and U.S. Pat. No. 8,111,458, all of which describe an array of retarders, which rotate different parts of the beam into different linear polarizations, with different orientation angles for fast axis. While those depolarizers provide a good improvement in terms of limitation on size of beam, and bandwidth of coherent beams, they depolarize the beams in a very limited way, due to being only a plurality of polarization rotators, and can work only in very specific wavelengths, which limit their usability considerably, as one would require having multiple devices in situations of tunable lasers, or broadband sources. The device described in U.S. Pat. No. 8,696,134 improves further by having segments that also have different phase delays (similar to our invention), but is complicated and costly to manufacture, as it requires significant work in order to produce it, and is therefore not practical. Additionally, by having the segments have different thicknesses, the design has significant side effects of reflections, scattering and diffraction effects, caused by the uneven surface of the device.
A Lyot and wedge type depolarizers are made by combining two birefringent materials, with different thicknesses, in a way that creates a spatially varying fast axis. First described in the 1930s, this type of depolarizer is commonly available today from many commercial providers, and is commonly used in achromatic systems, where the beams are very broadband, and large in physical size. As can be seen in the graph in FIG. 1 (DOP vs. Wavelength Bandwidth; laser beam wavelength of 1064 nm, with total length of 6 mm) the effectiveness of a Lyot type depolarizer greatly depends on the bandwidth of the beam. Typically, commercial vendors for Lyot depolarizers, specify the depolarizer as being effective only for beams with an optical bandwidth greater than 50 nm. FIG. 1 shows the effectiveness of a Lyot depolarizer as a function of the source's bandwidth. Note that lower degree of polarization (DOP) is desired.
However, a typical application that requires depolarization involves, for example, the use of a broadband source which is then spectrally filtered down to a narrow bandwidth beam, often by using a grating or other means of tunable spectral filtering. The narrow bandwidth beam, which often has a bandwidth in the order of 0.1 nm, is then applied to a sample to be tested for wavelength dependent transmission and reflection. The resulting beam is then detected to identify wavelength dependent behavior. This is then repeated hundreds or thousands of times, each time after tuning slightly the filtered wavelength, such that the spectral dependence of a sample can be obtained across thousands of individual wavelengths, in a broad range of wavelengths. Since the sample tested often has some dependence to polarization, it is required that the beam applied to the sample be depolarized. However, since the available depolarizers can only work for broadband beams, the existing solutions cannot provide a useful solution for such a case.
Some variations on the classic Lyot solution all contain two or more optical elements, which create some spatial variation to the polarization, such as wedge depolarizers. Those can decrease the minimum required beam size, but have a significant disadvantage of causing a beam deviation, due to the angle between the wedged glass elements, making this type of depolarizer not practical.
Another type of proposed solutions has arrays of half-wave waveplates, with different orientation axis, across the clear aperture. Examples include US patent US2008/0049321A1 and U.S. Pat. No. 8,111,458, in which a plurality of segments is proposed, each segment having a different fast axis orientation, and all segments sharing the same amount of birefringence, by having one homogeneous birefringent layer. In effect those solutions create an array of half-wave waveplates, where all the segments in this array act as half-wave waveplates, but with different angles of alignment for their respective fast axis. This in effect divides the incoming beam into multiple beams, and rotates the polarization axis of each such sub-beam going through each segment in a different amount. This allows the creation of arrays with very small segment sizes, and thus can significantly reduce the requirements of the minimum beam size that can be depolarized, as compare to the classic Lyot depolarizer. However, since these devices only vary the fast axis of each segment, but still use one layer of homogeneous birefringent material, it is effective only for cases where the value of birefringence equals exactly a half wave of the wavelength of the incident beam, as it can only effectively rotate the fast axis in the case that the wavelength of the beam applied to the device equals exactly twice the value of the half-wave of the device, dictated by the homogeneous birefringent layer. In other words, the above discussed devices act effectively as a depolarizer only for incident beams with a wavelength equal twice its birefringence value.
Another type of depolarizer that was proposed and discussed in the literature is a scattering type depolarizer. This depolarizer such as the one described in the paper titled “liquid crystal depolarizers” Journal of applied physics, Volume 90, number 15, October 2001, and a similar one described in U.S. Pat. No. 3,433,553, can create a depolarization effect, but with the cost of the side effect of scattering, and diffraction effects. Such side effects cause a portion the beam passing through the depolarizer to split out of the main beam, and continue to propagate in another direction, or a different angle. Since very often the beam to be depolarized is used for very accurate and tightly controlled measurements (such as the spectral measurement of a sample mentioned above), it is required to confine the beam to a tightly defined space, or area, and not have any parts of the beam scatter around and affect the measurement. This scattering effect is therefore very disruptive, and therefore such depolarizers with a scattering side effect are not commonly used.
U.S. Pat. No. 8,696,134 provides a further enhancement to the devices mentioned above, by adding an additional dimension of variation between the pixels of the array, in which the additional dimension provides a difference in phase delay between the different pixels. This in effect makes the device into having two variables: the difference in fast axis, and a difference in phase delay, which leads to a difference in birefringence between axes. This difference in birefringence makes the device an efficient depolarizer for multiple wavelengths, due to the multiple different birefringence values within the array. This in turn makes in a depolarizer that can handle small beams (due to the small pixel sizes), handle narrowband beams (due to variety of values within the pixels), but also handle beams with multiple wavelengths, or multiple beams with different wavelengths, or changing wavelengths, due to the variety of birefringence values across the clear aperture. This device is however extremely difficult and costly to manufacture, due to the required additional steps needed to create the different values of birefringence (by modifying the surface of the birefringent layer), and has significant side effects to the optical quality of the beam, primarily scattering, wavefront distortion, and diffraction, due to uneven surface of the device, needed to create the varying birefringent layer, in this method.
Therefore, there is a need to have a depolarizer that overcomes the above mentioned difficulties in the existing devices and yet can substantially depolarize a beam of light, regardless of the size of the beam, its center wavelength, optical linewidth, angle of incidence (or numerical aperture), or the coherence of the beam.